It is known to operate flyback converters at a fixed clock frequency, but the duty factor is regulated as a function of the output direct voltage. For this purpose, a voltage of a current sensor (usually a low-resistance measuring resistor), which is proportional to the current flowing through the primary winding, is compared to the deviation of the output direct voltage. As soon as the current flowing through the primary winding exceeds the value determined by the said deviation, the driven switch is opened. A triangular current is produced in the primary winding. An increase or decrease in the output direct current is compensated for by a decrease or increase, respectively, in the duty factor of the current flowing through the primary winding.
In order to limit the output direct current in the case of overload or short-circuit, in the prior art fixed-frequency flyback converters the output current is measured using a measuring resistor, the measured actual value is compared to a reference value and the difference resulting from this comparison is superimposed on the deviation of the output direct voltage when the output current exceeds a given value. This also results in a decrease in the duty ratio and consequently a current limitation. A diagram according to this state of the art is shown in FIG. 1. This current limitation solution is relatively complicated, as it requires a dedicated current regulator and a relatively expensive measuring resistor R.sub.2 capable of withstanding high loads in order to measure the output direct current I.sub.A.
A clock controlled converter with current limitation is shown in the European Patent No. 0 498 917, which is a "forward" type converter containing a first means for current limitation through current-dependent time reduction of the switch-on pulses, which operates in the range of the usual values of the current to be limited. This clock-controlled converter includes a second means of current limitation, which operates only in the upper range of the current to be limited and, depending on an output direct voltage, causes the switching frequency with which the switching pulses follow one another to decrease as the current increases.
Freely oscillating flyback converters, unlike fixed-frequency flyback converters, can be easily designed with automatic current limitation. Yet the well-known disadvantages of the freely oscillating flyback converters such as poor open circuit protection, undefined EMC relationships, etc., are unacceptable for most applications.
Therefore, one of the objects of the invention is to provide a flyback converter of the aforementioned type having all the advantages of a fixed-frequency flyback converter, and in which current limitation can be achieved in a simple and inexpensive manner.